Sensor for peracetic acid-hydrogen peroxide solution

ABSTRACT

Concentrations of stock hydrogen peroxide-peracetic acid solutions can be monitored resistively. The resistivity cell electrodes have titanium surfaces which resist corrosion better than other electrode materials including platinum electrodes. Such resistivity monitoring is particularly useful to verify the concentrations of sterilant stock solutions used in machinery for cleaning and sterilizing medical and dental equipment such as dialyzer reuse machines. Over the temperature range of interest the resistivity measurement is substantially not affected by changes of temperature in the room temperature range.

BACKGROUND OF THE INVENTION

It has generally become accepted that expensive apparatus particularlythat used in medical and dental applications need be recycled bycleaning and sterilizing before reuse. Much of the apparatus for suchpurposes requires very special handling in order to insure that it is infact sterile. It is often not practicable to use thermal means toaccomplish such sterilizing and therefore various chemical sterilizingagents have found use. It is of extreme importance that a high degree ofconfidence exist in the products that are going to be utilized for thesterilizing to insure that they have been properly mixed prior to beingactually used for the sterilization purpose. One normally starts with aconcentrate of the active materials to be used and dilutes them downwith water to the desired level for actual sterilization. It isimportant that the active material be neither over nor under diluted.

Sterilants of the peracetic acid-hydrogen peroxide types have been knownfor many years. The sterilants are prepared by mixing acetic acid andhydrogen peroxide to give an equilibrated solution of peracetic acid,acetic acid and hydrogen peroxide. When properly mixed and diluted,these solutions have shown great efficacy in killing not only bacteriabut various other microbiological materials. A great deal of literatureand patent material exist in dealing broadly with peraceticacid-hydrogen peroxide solutions for sterilization. Without limitation,the reader is directed to U.S. Pat. Nos. Bowing 4,051,058 and 4,051,059.The reader's attention is also directed to the text ORGANIC PEROXIDES byDaniel Swern and to M. G. C. Boldry, "The Bactericidal, Fungicidal andSporicidal Properties of Hydrogen Peroxide and Peracetic Acid," J. App.Bacteriology 54, 417-423 for further background on the field of the useof peroxides and hydrogen peroxide in microbiological applications.

In U.S. Pat. No. 4,517,081 there is described a machine which cleans andsterilizes dialyzers to enable their reuse. The machine includes meansfor passing the series of cleaning, sterilizing and rinsing fluidsthrough the compartments of the dialyzer in a predetermined sequence.Predetermined dilutions of stock solutions are accomplished in a fluidtank within the machine which communicates with a microprocessorcontrolled manifold for introducing controlled amounts of stocksolutions and water into the tank. Commercial embodiments of theinvention have utilized peracetic acid-hydrogen peroxide solutions ofdifferent concentrations for both the cleaning and sterilizingfunctions.

Peracetic-hydrogen peroxide solutions are inherently unstable and,therefore, subject to variation in concentration of active peroxycomponents with aging. Highly concentrated solutions are more stable.The high concentration of conventional commercial peraceticacid-hydrogen peroxide solutions must be manually diluted before thesolution can be conveniently handled as a stock solution in a machinesuch as described in U.S. Pat. No. 4,517,081. The pre-dilutionintroduces possibilities for human error in the dilution and, therefore,in the ultimate concentration and uniformity within the dilutedconcentrate of peracetic acid and hydrogen peroxide used to clean andsterilize the dialyzer. Should the diluted material be too dilute aserious risk of residual living bacteria remains. Too concentrated mayresult in insufficient rinsing to remove the sterilant and may evendamage the material being sterilized.

Conductivity measurements have been widely used to measureconcentrations of solutes in solvents and for measuring other parametersindicated by fluid concentration changes. For instance, bloodconductivity measurements have been used to monitor cardiac output asdescribed in U.S. Pat. Nos. 3,985,123, 4,572,206, 4,380,237, andBourdillon et al, Med. & Biol. Eng. & Comput. 17, 323-329 (1979).

While conductivity measurements to monitor concentrations of solutes inaqueous solutions are well known. It is believed that, heretofore, nosuch measurements have been used to monitor peroxide/peracid solutionsconcentrations.

SUMMARY OF THE INVENTION

Because of the possibilities of error in the dilution of stock solutionsof peracid-peroxide there exists a need to monitor the stock solution toassure that it is neither so strong that the final solution risks damageto the dialyzer or to the cleaning machine, and that it is not so weakthat it causes the final solutions to fail in their cleaning andsterilizing functions. The present invention accomplishes these purposesand provides a special electrode and electronic system for insuring thatproper mixing has taken place of a concentrate and purified water priorto its actual usage in the overall cleaning and sterilizing system.

This invention relates to a overall system for the monitoring ofblending of concentrates and water as a preliminary to actualsterilization of various attendant equipment. More particularly, it isused in conjunction with special electrodes found uniquely suitable forhydrogen peroxide containing solutions and for sterilants of theperacetic acid - hydrogen peroxide type.

As one aspect of the invention, therefore, there is disclosed a machinewhich automatically dilutes stock solutions of peracetic acid andhydrogen peroxide in a dilution tank for subsequent use as cleaning andsterilizing agents, the improvement comprising that the machine isprovided with a concentration sensing and warning means for determiningthe value of concentration dependent parameters of the stock solutionand warning if that value is outside a predetermined range.

In a further aspect of the invention there is provided a sensor formeasuring the resistivity of a peracetic acid-hydrogen peroxide solutioncomprising a spaced pair of titanium electrodes, resistivity measuringmeans communicating with said electrodes when said electrodes are incontact with the solution, said measuring means providing an outputindicative of the resistivity of the solution, comparison means forcomparing said resistivity indicative output with a predetermined rangeof acceptable outputs and issuing a warning signal when saidconductivity indicative output is outside said range of acceptableoutputs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a portion of a dialyzer cleaningand sterilizing machine incorporating the sensor of the invention.

FIG. 2 is a schematic representation of the preferred embodiment of theresistivity measuring circuit.

FIG. 3 is a graph showing the resistance of a solution withoutstabilizer useful in the present invention as a function of thepercentage of hydrogen peroxide in the solution.

FIG. 4 is a plot of the change in resistivity of six different solutionsof 21% dilution as a function of temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is depicted schematically a portion of thepreferred embodiment of the invention wherein the resistivity sensor ofthe invention is employed in a machine which automatically dilutes stocksolutions cleaning and sterilizing fluid and circulates them forcleaning an article such as a dialyzer or other medical or dentalapparatus. Fluid flow directions in FIG. 1 are indicated by the arrows.

A stock solution of hydrogen peroxide-peracetic acid is held in acontainer 10 which communicates with a dilution tank 18 by means of aconduit 15. A valve 30 and conduit 15 operate to control the flow of thestock solution into the dilution tank 18.

The valve is controlled by a microprocessor which opens and closes thevalve in accordance with a predetermined program for controlling thecirculation of fluids through the machine. Spaced probes 35 in conduit15 upstream of the valve 30 form a resistivity cell for sensing theresistivity of the fluid passing thereby. Probes 35 are electricallyconnected to a resistivity measuring circuit 40 which issues an outputto the microprocessor as described in detail below.

The tank 18 is also provided with a connection to a vacuum source 19, afluid outlet 20 communicating with the article or articles to be cleanedand a water inlet 22. Valves not shown in each of the respective inletsand outlets are also suitably provided and controlled by themicrocomputer. The tank sits on a load cell 25 whose output is also fedinto the microcomputer. The load cell 25 allows monitoring of the volumeof the fluids in dilution tank 18 so that preprogrammed dilutions andfluid circulations may be performed by the microcomputer. For furtherdetails on the construction and operation of such a machine, the readeris referred to U.S. Pat. No. 4,517,081.

The resistivity measuring circuit preferably utilizes a constant voltagesource. FIG. 2 depicts the elements of the preferred circuit. A constantvoltage source 41, from a 1KHz sine wave oscillator applies a voltageacross the bipolar electrode probe assembly 42 through a Wheatstonebridge 43. Suitably the bipolar probe assembly 42 comprises the twoelectrodes 35 of FIG. 2. The output of the Wheatstone bridge is fed to adifferential amplifier 44 which is provided with a gain of 10 and whichconverts the sensed current in the bridge circuit to a voltageproportional to the resistance across the probe assembly. The amplifiedsignal is conditioned by feeding through a band pass filter 45,desirably a 2-pole filter having a center frequency of 1000 Hz and 3dbpoints of 500 and 1500 Hz. The conditioned signal is then sent to arectifier 46. Suitably, the rectifier 46 is a full wave rectifiercircuit having a voltage gain of two and having its gain temperaturecompensated. The rectified signal is then fed to a non-invertingamplifier 47 which has a gain of 2 and has a capability of offsetadjustment. Next the signal passes through an analog-to-digitalconverter 48 to provide an eight-bit digital converted signal. Theconverted signal is ultimately fed to a microcomputer 9 where it iscompared to a preprogrammed range of acceptable signal values for thestock solution. If the measured signal is within the preprogrammed rangethe machine will continue to operate normally. If outside the acceptablerange, a machine error is indicated and an alarm signal is issued.Suitably the alarm signal triggers an audible or visual alarm 50 andalso triggers an automatic shutoff of the valve 30 to stop flow of thestock solution into the cleaning machine.

In attempting to develop a resistivity sensor for the peroxide/peracidsystem it was discovered that electrode metals such as stainless steeland other metals and, in deed, even conventional platinum orplatinum-black electrodes were rapidly corroded by the solution and,therefore, unsuitable. Signs of corrosion appeared in a matter ofminutes. However, it has unexpectedly been discovered that titaniummetal electrodes are sufficiently inert to allow their use in relativelyconcentrated peroxide/peracid solutions. Titanium electrodes showed nosigns of corrosion after weeks of immersion.

The electrodes are preferably titanium having a configuration of roundwith hemispherically shaped ends. The electrodes may conveniently be0.25" in diameter, and spaced from center to center by about 1.0".

The resistance of the subject solutions is relatively constant over afairly wide range of temperatures including normal ambient temperaturesof about 18°-25° C. Consequently, little or no correction need to bemade for normal temperature variations.

It is recommended that a calibration test be seen for the specificperacetic acid hydrogen peroxide mixture to be used to establish thecurves for concentration limits that will be used to set the alarmlevels in the microcomputer.

In operation a commercial peracetic acid-hydrogen peroxide concentrate(which may be, for example, Renalin® available from Minntech Corporationof Minneapolis, Minn. is prediluted to a stock solution in the container10. Renalin® has a nominal composition in the concentrate of about 23%of H₂ O₂, 5% peracetic acid and 9% acetic acid balance H₂ O. Thedilution tank 18 is tared with load cell 25. Valve 30 is then opened toallow the stock solution to be drawn into the dilution tank by means ofthe vacuum source 19. As the stock solution flows past the electrodes 35the resistance of the solution is checked by the computer to verify thatit is within the preprogrammed range of acceptable values. If within theacceptable range, the stock solution is permitted to continue flowinginto the dilution tank until a predetermined amount has been drawn in asindicated by the load cell, at which point the valve 30 is closed. Wateris added to tank 18 by means of water inlet 22 to dilute the stocksolution to a hydrogen peroxide concentration of 1.7% or 3% depending onwhether the solution is to be used respectively for initial cleaning orbactericidal sterilizing operations.

If substantial deviations are encountered in the output of theresistivity beyond those calibrated as acceptable the measuring circuittriggers the microcomputer to interrupt the dilution sequence early,closing valve 30 and triggering alarm 50 to alert the operator of theerror condition. The stock solution can the be replaced, the dilutiontank flushed and the dilution operation restarted.

The invention may be illustrated by the following non-limiting examples.

EXAMPLES

In the following examples stock solutions suitable for use in cleaningand sterilizing a dialyzer reuse machine as described above wereprepared by diluting the concentrates such as Renalin® in the examplesto 21% of their concentrate strength with highly purified water (2liters concentrate made up to 9.46 liters (2.5 gal.)). Resistancemeasurements were made on the stock solutions.

EXAMPLE I

Stock solutions were prepared from the concentrates as referred to aboveand resistances measured with a sensor of the invention. FIG. 3 is aplot of the measured resistance as a function of the initial peroxideconcentration. The substantially linear relationship demonstrates that amixture of peracetic and peroxide concentration can be successfullymonitored resistivity in this complex solution.

EXAMPLE II Temperature and Aging Dependence

Hydrogen peroxide-peracetic acid stock solutions were used in thisexperiment. The stock solutions were prepared for the followingingredients:

Renalin® diluted to 21% of concentrate to a stock solution. These weretested as freshly made and after standing for several days and arelabeled E fresh and E aged.

Other formulations having compositions of the following were made andtested:

A. 30% H₂ O₂, 12.85% HOAc to give peracetic acid concentration of about5%.

B. 27% H₂ O₂ ; 12.85% HOAc to give peracetic acid concentration of about5%.

C. 24%, H₂ O₂ ; 12.85% HOAc to give a peracetic acid concentration ofabout 4%.

The resistances of stock solutions prepared as described above weremeasured and plotted as a function of temperature on both fresh and agedstock solution using a resistivity cell having titanium electrodes asdescribed previously. As can be seen in FIG. 4, the plot of theresistivity as measured was substantially flat showing the virtual lackof temperature dependence over a room temperature range of about 15° C.to 26.6° C. The upper curves are for Renalin® concentrate as a startingpoint and the lower curves are for formulations of A, B and C.

While the invention has been described above with regard to thepreferred embodiment, it will be readily apparent that the invention isnot so limited. For instance the sensor may just as usefully be employedin a machine for cleaning and sterilizing dental or surgical equipmentor the like. These and other modifications within the skill of those inthe art should be considered within the scope of the invention which islimited only by the language of the following claims and their legalequivalents.

What is claimed is:
 1. In a machine which automatically dilutes stock solutions of peracetic acid and hydrogen peroxide for subsequent use as cleaning and sterilizing agents, the improvement comprising:a concentration and sensing means operably in contact with the stock solution prior to use of the stock solution as cleaning and sterilizing agents for determining a resistively measured value of a composition peroxide concentrate dependent parameter of the stock solution and issuing an alarm signal if that value is outside a predetermined acceptable range.
 2. A machine as in claim 1 wherein said concentration and sensing means includes a resistivity measuring cell in contact with the stock solution and having a pair of titanium electrodes spaced apart from one another by less than 3 centimeters.
 3. A machine as in claim 1 wherein said concentration and sensing means comprises:a resistivity measuring cell in contact with the stock solution and having a pair of electrodes; resistivity measuring means for communicating with said electrodes and providing as an output said value indicative of the resistivity of the solution; and comparison means for comparing said resistivity indicative output with said predetermined acceptable range and issuing said alarm signal when said resistivity indicative output is outside the acceptable range.
 4. A machine as in claim 3 wherein said resistivity measuring means comprises a constant voltage source which applies a constant voltage to said electrodes through a bridge circuit, the bridge circuit providing a signal which is fed sequentially through a differential amplifier, a band pass filter, a rectifier, a non-inverting amplifier and an analog to digital converter to yield a digital signal indicative of the resistivity of said solution.
 5. A machine as in claim 4 wherein said digital signal is said resistivity indicative output, and said comparison means comprises a microcomputer preprogrammed with said predetermined range of acceptable output signals.
 6. A machine as in claim 1 further comprising shut down means responsive to said alarm signal for interrupting operation of the machine when said alarm signal is issued.
 7. A machine as in claim 6 having a stock solution tank for holding said stock solution prior to dilution, a dilution tank wherein said dilution is performed and a conduit between said tanks, the shut down means comprising a valve in said conduit which is controlled to close in response to said alarm signal. 